The invention relates to a device for changing the length of a running path of an electromagnetic wave according to the features specified in the introductory part of claim 1.
Devices of this type are known and serve for the running time change of an electromagnetic wave, in particular a light beam, for producing a Doppler shift, for producing a time-dependent spectrum. They are applied for example with optical coherence tomography (OCT), in fiber gyroscopes or also with Fourier transform spectroscopy. Such devices may be applied everywhere where it is necessary to change the optical path length of a beam path.
Known devices function either with mirror arrrangements or also as discussed here with a wave guide, in particular a quartz fiber whose length is changed by extending the guide or the fiber.
In the U.S. Pat. No. 5,321,501 there is described an apparatus for optical coherence tomography which contains such a device. This device comprises a fiber-optic wave guide which is wound around a piezoelement which extends on application of an electric current, by which means the wave guide wound thereon also extends, by which means the running path of the light changes. A disadvantage of the devices described there is that the extension of the fiber optic guide effected on account of the piezoelectric extension is not effected in a defined manner and is in several axes, by which means double refraction induced by voltage may occur, which leads to an adulteration of the results. Furthermore the device described here in dynamic operation tends to partially oscillate, which questions its usability.
From JP 600 63 517 there is known a similar phase modulator with which piezoelectric elements are arranged at four sides of an essentially cylindrical body which are activatable simultaneously. With this arrangement although a comparatively high extension may be achieved, however the extension is effected in an undefined manner an in several axes, so that the previously mentioned disadvantages are also given here.
The problem of the partial oscillations has been recognised. In U.S. Pat. No. 5,101,449 there is met an arrangement with which in the piezoelement eccentrically there is provided a recess in order to reduce these disadvantageous effects. Also from U.S. Pat. No. 5,493,623 there is known a device which is to avoid these effects, in particular the succeptibility to resonances.
In U.S. Pat. No. 5,867,268 there is described an apparatus for optical coherence tomography with which a light wave guide is adhesed onto a disk-shaped piezoelement in a spiral-shaped manner. From U.S. Pat. No. 5,029,978 there is known a device with which the light wave guide is cast into a piezoelectrically active polymer. In U.S. Pat. No. 5,135,295 there is described the coating of an optical fiber with piezoelectrically active material in order in this manner to be able to stretch the fiber practically directly.
Common to all previously mentioned devices is the fact that the optical phase shifting is changed via a voltage applied to the piezoelement. The voltage applied to the piezoeelement is however only proportional to its length changes in the first approximation since piezoelements on acount of the system have a non-linear extension behaviour. With a dynamic activation, i.e. with an activation with a periodically changing voltage, piezoelements at certain intrinsic frequencies tend to resonate. With certain applications for optical phase modulators it is necessary to know the exact path length change of the phase modulator in order to empirically compensate the non-linear behaviour of the piezoelements or to be able to construct a control loop in order to create the desired path change.
Against this state of the art it is the object of the invention to design a device of the known type such that the previously mentioned disadvantages are largely avoided and that in particular a defined path length change is achievable which where appropriate may also be recorded with regard to measuring technology.
This object is achieved according to the invention by the features specified in claim 1. Advantageous embodiments are to be deduced from the dependent claims, the subsequent description and the drawings.
The basic concept of the present invention is to create the extension of the wave guide only where this runs essentially in a straight line in order to ensure a defined length change whilst avoiding the initially mentioned disadvantages. The wave guide is with this wound up on a divided multi-part core and the length change is effected by distance change of the core parts to one another. In the simplest form this may be effected in that the core is formed of two halves whose distance to one another is changeable. In this manner it is possible to impinge the wave guide in its straight-lined sections between these halves almost exclusively with tensile forces in order to achieve a defined length change. At the same time there may be applied practically any drive for the extension. A measuring of the length change is also possible without further ado in that the distance change of the halves to one another is determined.
Basically it is conceivable to pivot the halves or core parts to one another about an axis in order by way of this to create a position change. Preferably the position change however is effected by way of a linear displacement of the core parts to one another since then it may be ensured that with a suitable arrangement of the windings as well as design of the cores, exclusively tensile stresses are exerted onto the wave guide. By way of this one may avoid the voltage induced double refractions which are otherwise so disadvantageous. At the same time it is basically of no importance whether both core parts or several core parts are moved to one another or also whether only one core part is moved from the other or from the remaining core parts. The extension may at the same time be effected by way of a piezoelement, by way of a stack of piezoelements, electromagnetically, electromotorically (with an eccentric drive) or with another suitable drive, without departing from the concept according to the invention.
In order to produce chiefly tensile stresses in the wave guide it is useful to correspondingly mount the core part to be moved or the core parts to be moved. This may for example be effected by a linear guide, e.g. a rolling bearing guide, by way of a sliding bearing guide or also by way of a pneumatic or hydraulic guide. Advantageously here also a solid body joint may be applied, for example a parallel leaf spring guide.
It is particularly useful when the core comprises sections along which the wave guide runs curved and other sections along which the wave guide runs essentially in a straight line, and the wave guide in the regions in which it runs curved are fixedly adhesed on the associated core section or is fastened with a material or friction fit in another suitable manner so that it is ensured that in these regions there is effected no or only a very slight relative movement between the core and the wave guide, but only in the regions in which the wave guide runs in a straight line. In the latter regions the wave guide should bear preferably not directly and if so then only with as small as possible coefficient of friction, in order not to introduce any transverse forces with the tensile stresses. On the other hand such a supporting of the sections of the wave guide which run essentialy in a straight line brings with it considerable advantages, in particular in the dynamic operation, since then an uncontrolled oscillation of the wave guide in these regions may be prevented.
It is particularly advantageous when the core sections in which the wave guide runs curved are formed by halves of a cylindrical body which is divided along its axis. In the remaining region between these cylinder halves there are usefully provided guides which prevent or at least dampen transverse oscillations of the wave guide.
With the device according to the invention preferably also a piezoelement or a stack of piezoelements may be applied as a drive, since the non-linear behaviour may either be compensated by a suitable control or by way of measuring technology. For this, parallel to the drive, e.g. between the core sections movable to one another there is to be provided a recording device, preferably a capacitative recorder. However also according to the type and the scope of the extensions any other recording device, e.g. wire strain gauge or inductive recorder may be applied.
As wave guides advantageously quartz fibers may be applied, wherein according to the demands, protective-coated or also non-protective-coated quartz fibers may be used. Such quartz fibers are inexpensive to manufacture and may be elastically extended up to 10% so that already with a low winding number there may be achieved large length changes. Preferably the winding is effected in one layer, however also multi-layered windings may be provided, when it is ensured that the wave guide is fixed in the region of its curved sections, in order to avoid multi-axis initiations of tension.
Preferably the device comprises a control which in dependence on the signal emitted from the distance recorder (recording device) controls drive elements determining the distance of the core parts so that the length change of the running path is changeable in a directed manner, in particular also in dependence on time. In this manner the length change may be effected according to a pregiven speed or path profile.
With the application of a piezoelement as a drive it may be advantageous to operate this in the resonance region, since then the force development, i.e. thus the efficiency is particularly high. Furthermore the device in this region functions in a very frequency-stable manner.
A preferred application purpose of the device according to the invention is with Fourier transfrom spectroscopy, and specifically in the sample branch. Also the device may be applied as a phase modulator selectively in the reference or in the sample branch of an interferometer which is applied with optical coherence tomography. Such an interferometric measuring arrangement may for example comprise two coupling means wherein in a first coupling means the light waves coming from a light source are coupled into a sample arm and a reference arm and in the second coupling means the light waves coming from the reference arm as well as the light waves coming from the sample arm are superimposed and led to at least one, preferably two detectors which convert the optical interference into an electric signal. With this arrangement the reference arm is formed by a fiber guide extending between the coupling means. The sample arm comprises preferably at least two fiber guides which extend to both sides of the first coupling means. Such an arrangement is particularly strong in light and on account of its construction (guiding of the light waves almost exclusively in fiber guides) is largely insensitive to external surrounding influences. Via the free end of the sample arm furthermore the topology of an external object may be covered, as is for example effected with Fourier transform spectroscopy. Advantageously the device according to the invention is applied in the sample branch as well as in the reference branch of an interferometer, wherein one device serves the dynamic path length change of the light and the other for the static setting of the operating point. With this in an advantageous embodiment of the invention both devices may be thermally connected to one another, for example by way of a heat-conducting common carrier frame, by which means it is ensured that also with a changing operating temperature there is effected no displacement of the operating point outwardly appearing, or a corresponding displacement is effected also in the other branch which compensates this.
On account of the defined length change or the simple recordability of the length change the device according to the invention may be applied in many fields, for example for field strength measurement whilst exploiting the electrostrictive effect.